1. Field of the Invention
The presently disclosed instrumentalities pertain to thermochromic pigments in materials and articles that may be worn by a person or applied to a substrate. In particular, the pigments have a color activation temperature ranging from about 40° C. to 45° C.
2. Description of the Related Art
Chemicals that change color over a range of temperatures are known as thermochromic systems. Thermochromic chemicals can be manufactured to have a color change that is reversible or irreversible. U.S. Pat. No. 5,591,255, entitled “Thermochromic Ink Formulations, Nail Lacquer and Methods of Use,” issued Jan. 7, 1997, to Small et al., discloses methods of producing thermochromic coating formulations without ingredients known to be harmful to thermochromic inks. The use of distilled water as a fountain solution for off-set printing using thermochromic ink is also disclosed.
Thermochromic systems use colorants that are either liquid crystals or leuco dyes. Liquid crystals are used less frequently than leuco dyes because they are very difficult to work with and require highly specialized printing and handling techniques. Thermochromic pigments are a system of interacting parts. Leuco dyes act as colorants, while weak organic acids act as color developers. Solvents or waxes variably interact with the leuco dyes according to the temperature of the system. As is known in the art, thermochromic systems are microencapsulated in a protective coating to protect the contents from undesired effects from the environment. Each microcapsule is self-contained, having all of the components of the entire system that are required for the color change. The components of the system interact with one another differently at different temperatures. Generally, the system is ordered and colored below a temperature corresponding to the full color point. The system becomes increasingly unordered and starts to lose its color at a temperature corresponding to an activation temperature.
Below the activation temperature, the system is usually colored. Above the activation temperature the system is usually clear or lightly colored. The activation temperature corresponds to a range of temperatures at which the transition is taking place between the full color point and the clearing point. Generally, the activation temperature is the temperature at which the human eye can perceive that the system is starting to lose color, or alternatively, starting to gain color. Presently, thermochromic systems are designed to have activation temperatures over a broad range, from about −20° C. to about 80° C. or more. With heating, the system becomes increasingly unordered and continues to lose color until it reaches a level of disorder at a temperature corresponding to a clearing point. At the clearing point, the system lacks any recognizable color.
In this manner, thermochromic pigments change from a specific color to clear upon the application of thermal energy or heat in a thermally-driven cycle exhibiting well-known hysteresis behavior. Thermochromic pigments come in a variety of colors. When applied to a substrate, such as paper, the pigment exhibits the color of the dye at the core of the microcapsules. In one example, when heat is applied generally in the range of 30 to 32° C., the ink changes from the color of the pigment to clear. When the substrate is allowed to return to a temperature under approximately 30° C., the ink returns to the original color of the pigment.
Temperature changes in thermochromic systems are associated with color changes. If this change is plotted on a graph having axes of temperature and color, the curves do not align and are offset between the heating cycle and the cooling cycle. The entire color versus temperature curve has the form of a loop. See generally FIG. 1 where the extent of color change presents a gap 100 that differs between color change that occurs upon heating 102 to an ultimate clearing point 104 versus cooing 106 to an ultimate full color point 108. This shows that the color of a thermochromic system does not depend only on temperature, but also on the thermal history, i.e. whether the particular level of color was reached during heating or during cooling. This phenomenon is generally referred to as a hysteresis cycle and specifically referred to herein as color hysteresis or the hysteresis window. Decreasing the width of this hysteresis window to approximately zero allows for a single value for the full color point and a single value for the clearing point. This would allow for a reliable color transition to be observed regardless of whether the system is being heated or cooled.
Prior art reveals that the color transition range of microencapsulated thermochromic systems may be adjusted by shifting the full color point upward toward the clearing point, or shifting the clearing point downward toward the full color point, as explained in U.S. Pat. No. 6,494,950. These shifts are accomplished by adding high melting point materials to increase the full color point or, alternatively, by adding low melting point materials to the system to decrease the clearing point. Thus, the full color point or clearing point may be lowered or raised, but the overall temperature range between the two points remains unchanged because the amount of separation or width across the hysteresis window is left largely unaffected.
Specific thermochromic coating formulations are known in the art. See, for example, U.S. Pat. Nos. 4,720,301, 5,219,625, 5,558,700, 5,591,255, 5,997,849, 6,139,779, 6,494,950 and 7,494,537, all of which are expressly incorporated herein by reference. These thermochromic coatings are known to use various components in their formulations, and are generally reversible in their color change. Thermochromic; pigments for use in these coatings are commercially available in various colors, with various activation temperatures, clearing points and full color points. Thermochromic coatings may be printed by offset litho, dry offset, letterpress, gravure, flexo and screen processes, among other techniques
Microencapsulated thermochromic pigments may be formulated on commercial order and incorporated in coatings that change color in response to changes in temperature. By way of example, U.S. Pat. No. 5,281,570, issued to Hasegawa et al., teaches how to form microencapsulated thermochromic pigments that may be used directly or microencapsulated. The microencapsulated system is preferred and includes a leuco dye system that is mixed with longer chain alcohols, caprates, stearates, palmitates, etc., that are selected for melting point to control the color transition temperature of the pigments. These materials form the core of a microcapsule that may be microencapsulated by a wall of resin, such as an amine resin. Microencapsulated thermochromic pigments having a variety of colors and color transition temperatures may be purchased on commercial order from suppliers, such as Chromatic Technologies, Inc. of Colorado Springs, Colo.